Instructions of a program to execute memory operations, such as (e.g., load and store) as well as a sequence of instructions (e.g., threads and work items) of the program are typically not executed in their specified order. Some operations, such as memory fence operations (e.g., load-acquire and store-release), atomic operations appearing to occur instantaneously (e.g., complete in a single step relative to other threads) and locks are used to synchronize the memory operations in multi-threaded environments (e.g., environments using a graphics processing unit (GPUs) to process many tasks in parallel) or when interfacing with other hardware (e.g., via memory buses).
For example, memory fence operations or barriers provide an ordering constraint on the memory operations issued before and after an instruction so that stores issued prior to the fence are visible before loads issued after the fence. The efficiency of memory synchronization operations depends on a wide variety of criteria.